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Hydrogen sulfide, oxygen, pH

I have a 75 gal planted tank. After changing the
UV bulb in the sterilizer fed by one of the
filters, I forgot to open one of the four valves
and left it like that for about three weeks. When
I was under the tank looking at something else, I
noticed that it was closed and stupidly opened it.
The smell of rotten eggs, I suppose hydrogen
sulfide, quickly filled the tank. I immediately
did a 75% water change and completely changed the
media but by the next morning fish were gasping
for oxygen and the pH was low even though the
water no longer smelled. I turned off the CO2
injection (it does not run at night anyway) and
pulled a filter return out of the water so it
would aerate and did this for the next two weeks.

I did another 75% water change two weeks later. A
week after that pH was up to 7.2 with aeration so
stuck the return back under water (I have soft
water). Left the CO2 off. The next morning the
pH was 6.4 and three rainbows had died and the
other rainbows were gasping (rainbows and SAEs
seemed most prone to oxygen deprivation - angels,
tetras, and catfish were fine.). Even with
aeration and no CO2, the pH drops during the night
to around 6.7 and rises to 7.2 during the day.
Before the incident, I kept the pH around 6.6 to 6.7
all the time and everything was fine, but now it seems
that 6.7 means too little oxygen. Is this still
the effects of the hydrogen sulfide? Would adding
carbon to one of the filters or adding some
aquarium chemical help?

I have a 75 gal planted tank. After changing the
UV bulb in the sterilizer fed by one of the
filters, I forgot to open one of the four valves
and left it like that for about three weeks. When
I was under the tank looking at something else, I
noticed that it was closed and stupidly opened it.
The smell of rotten eggs, I suppose hydrogen
sulfide, quickly filled the tank. I immediately
did a 75% water change and completely changed the
media but by the next morning fish were gasping
for oxygen and the pH was low even though the
water no longer smelled. I turned off the CO2
injection (it does not run at night anyway) and
pulled a filter return out of the water so it
would aerate and did this for the next two weeks.

I did another 75% water change two weeks later. A
week after that pH was up to 7.2 with aeration so
stuck the return back under water (I have soft
water). Left the CO2 off. The next morning the
pH was 6.4 and three rainbows had died and the
other rainbows were gasping (rainbows and SAEs
seemed most prone to oxygen deprivation - angels,
tetras, and catfish were fine.). Even with
aeration and no CO2, the pH drops during the night
to around 6.7 and rises to 7.2 during the day.
Before the incident, I kept the pH around 6.6 to 6.7
all the time and everything was fine, but now it seems
that 6.7 means too little oxygen. Is this still
the effects of the hydrogen sulfide? Would adding
carbon to one of the filters or adding some
aquarium chemical help?

Sounds to me like you KH has dropped significantly. A swing between 6.7 &
7.2 would tend to indicate there is almost no buffer left. Suggest you add
some Sodium Bi-carb to stabilise it before you get a Ph Crash.

1. EYE CONTACT: If liquid is splashed into eyes, or if irritation of the eye develops after exposure to H2S open victim's eyes while under gentle, lukewarm, running water.
Use sufficient force to open eyelids. Have victim "roll" eyes. Minimum flushing is for 15 minutes. Victim must seek immediate medical attention from an ophthalmologist.

2. INGESTION: Ingestion is an unlikely route of exposure for Hydrogen Sulfide.

3. INHALATION: Remove victim(s) to fresh air, as quickly as possible. Trained personnel should administer supplemental oxygen and or cardio-pulmonary resuscitation, if necessary.

4. SKIN CONTACT: If liquid is spilled on skin, or if irritation of the skin develops after exposure to liquid or gas, immediately begin decontamination with running water. Minimum flushing is for 15 minutes.
* Remove exposed or contaminated clothing, taking care not to contaminate eyes. Victim must seek immediate medical attention.
* In case of frostbite, place the frostbitten part in warm water.
* DO NOT USE HOT WATER. If warm water is not available, or is impractical to use, wrap the affected parts gently in blankets.
* Alternatively, if the fingers or hands are frostbitten, place the affected area in the armpit.
* Encourage victim to gently exercise the affected part while being warmed. Seek immediate medical attention.

5. NOTES TO PHYSICIANS:Administer oxygen, if necessary and treat symptoms. Be observant for initial signs of pulmonary edema.